I assume you're talking about Gmail (you didn't say). When sending email, it can be marked low, medium, or high priority. I believe a contact with one of these markings would get an email to him/her so marked.
I've noticed that Google Contacts provides a field called "Priority" but they don't tell you what that field modifies. Does anyone know what the practical result is of assigning low, medium, or high "priority" to a contact in Google Contacts? I've spent 20 minutes searching the web for a single-sentence clear answer, but no dice. Someone's got to know.